Workers at Consignia are threatening to stage a national postal strike over a £1bn plan to sell part of its facilities department to construction group Balfour Beatty.

The Communication Workers Union (CWU) is this weekend preparing to ballot its 180,000 Consignia members over industrial action that could lead to a series of one- day walkouts next month.

The decision ends a week's exhaustive negotiations with Consignia management over plans to transfer 7,000 postal workers to a proposed new company, 49 per cent owned by Balfour Beatty. Insiders said that despite Consignia offering to delay the deal until September to allow further talks, CWU representatives believed the terms offered to staff were insufficient.

The news adds to a new wave of union activism sweeping Britain. On top of a potential postal strike, the Trades Union Congress (TUC) is planning to launch a major campaign on executive pay, starting at Vodafone's annual general meeting a week on Wednesday.

If CWU members vote to strike, it will bring the postal system to a halt, as 90 per cent of Consignia employees are union members. It would be a considerable blow to Allan Leighton, Consignia's chairman, who is desperate to restore to harmony to the troubled group.

A Consignia spokesman said: "We are keen to finalise things with Balfour Beatty. This really is a Rolls-Royce deal for the company and employees. We have built into the deal a high degree of personal protection for the employees – continued union representation and retention of the Consignia pension plan." However, the CWU wants assurances that Consignia won't sell its stake in the new joint company, known as Romec, to Balfour Beatty for at least seven years.

The CWU believes a sale would affect its members' rights to stay in the Consignia pension scheme. It also wants guarantees that should Romec make any redundancies then its members would be offered alternative employment within Consignia. It is understood that in a letter to be sent this week to its members urging them to vote for strike action, the union will say that staff are being "herded" into a joint venture and treated as though they were "office furniture".

Meanwhile, Vodafone is set to face the wrath of the TUC at its 31 July AGM. The union body is urging its 630 pension trustee members to vote against Vodafone's remuneration policy. The TUC wants its trustees to follow the guidance produced by corporate governance consultancy Pirc, which says performance targets set for Vodafone chief executive Sir Christopher Gent are too low.

The TUC's trustees wield considerable power, controlling £260bn of funds, around a third of Britain's occupational pension funds.

Tom Powdrill, the TUC's institutional investment officer, said: "This vote is the first serious test for the Government's plans to hold directors to account for executive excess." The TUC was planning to make two more protests on executive pay this year, he said.